Method and apparatus for controlling rush-drag in a paper machine

ABSTRACT

A system for controlling the difference between the velocity of a pulp slurry discharging from a paper machine headbox and the speed of the fourdrinier wire onto which said pump slurry is deposited. The system includes means for deriving a signal representative of the aforementioned speed differential thus allowing a direct comparison between the actual velocity differential and the desired velocity differential. The control system is based upon a differentiated and rearranged form of the equation v2 2gh which is dv (g/v)dh, wherein dv the rush or drag in units of speed, v the wire speed, g the gravitational constant, and dh the differential head, i.e., that part of the total head which gives rise to the velocity difference between the slice velocity and the wire speed, in units of inches of H20.

United States Patent [151 3,661,701

Al-Shaiklt May 9, 1972 [54] METHOD AND APPARATUS FOR OTHER PUBLICATIONS CONTROLLING RUSH-DRAG IN A Pearson; Automatic Headbox Operation, Tappi. Vol. 46, PAPER MACHINE No. 10. oct. 1963, p. l72A- 195A [72] Inventor: Abdul-Rahman A. Al-Shaikh, Mount Kisco. Primary Eraminer-S. Leon Bashore Assistant Examiner-Alfred A. DAndrea, Jr.

[73] Assignee: westvaco Corporation, New York NY Attornev-Alfred L. Michaelsen and Robert S. Grimshaw [22] Filed: June 23, 1970 [57] ABSTRACT [2 l I Appl. No.: 49,067 A system for controlling the difference between the velocity of a pulp slurry discharging from a paper machine headhox and the speed ofthe fourdrinier wire onto which said pump slurry Cl I62/263 is deposited. The system includes means for deriving a signal l 1fl/06 representative of theaforementioned speed differential thus [58] Field of Search 162/259, 198. 263, 252 allowing a direct comparison between the actual velocity differential and the desired velocity differential. The control 56] Referen e Cited system is based upon a differentiated and rearranged form of the equation v 2g]: which is dv (g/r)dlt, wherein til the UNITED STATES PATENTS rush or drag in units of speed, v the wire speed. g the gravitational constant. and dh the differential head. i.e. that 3077924 2,1963 Eastwood "162,259 part of the total head which gives rise to the velocity dif- 3490689 1/1970 Han et "162/252 x ference between the slice velocity and the wire speed. in units of inches of H 0. l

9 Claims, 1 Drawing Figure LEVEL CONTROL 23 SYSTEM @l ll l 31/ PRSES. 32 62 TRAN M/TTE)R 52 66 65 $2 5 2 7131' r 36' CONTROLLER 4 54 64b L 73 5/ 50 64 J J:5 70

a A P I v /z TRANSMITTER g SQUARER 3 fidH 46 i 7 756 6 41 RD 42 CONTROLLER 4 44 43 METHOD AND APPARATUS FOR CONTROLLING RUSH- DRAG IN A PAPER MACHINE BACKGROUND OF THE INVENTION l Field to Which the Invention Pertains In the manufacture of paper, a dilute aqueous slurry of cellulose fibers is deposited on a moving foraminous belt referred to as the fourdrinier belt or wire. When the water drains away, the cellulose fibers in the slurry form the resulting paper sheet.

In order to obtain a sheet which has the desired specifications, e.g., thickness and basis weight, it is imperative that one control the quantity and velocity of the slurry exiting from the opening or slice of the paper machine headbox. Moreover, the relationship between slice velocity and the speed of the fourdrinier wire determines how successive layers are built up in the sheet. If wire speed is appreciably less than stock or slice velocity, the fibers will roll, resulting in forward waves across the sheet. If wire speed is greatly in excess of stock velocity, backward waves would be formed across the sheet, and a combing effect on the fiber results. When uniform fiber distribution is desired, the well established principle is to maintain slice velocity close to wire speed. If the difference between stock velocity and wire speed is more than 20 or 30 f.p.m., it is difficult to make acceptable paper.

The difference between wire speed and slice velocity must be chosen with regard to the properties desired in the finished paper. For example, if the wire speed is in excess of the slice velocity, a high proportion of the fibers will be drawn into the machine direction. This result is desirable in certain grades of paper, e.g., spinning and twisting papers.

In the paper industry, the term rush-drag is used to refer to the difference between slice velocity and wire speed. If the slice velocity is in excess of the wire speed, it is said that the stock is "rushing the wire. Alternatively, if the slice velocity is less than the wire speed, it is said that the stock is dragging the wire. A rush-drag of zero indicates that the velocity of the stock discharging from the paper machine headbox, i.e., the slice velocity, is exactly equal to the wire speed. A rush of f.p.m. means that the slice velocity is 10 f.p.m. greater than the wire speed. A drag of 10 f.p.m. means that the slice velocity is 10 f.p.m. less than the wire speed.

As previously indicated, an excessive rush or drag cannot be tolerated. However, it is quite common for a papermaker to desire that his system operate with a relatively small rush or drag, depending upon the grade of paper to be manufactured. Although the desired rush or drag is quite critical, it is also often quite small in absolute value. For example, a rush or drag of ten feet per minute might be desired. One can appreciate the magnitude of the problem of controlling this relatively small speed differential when it is understood that paper machines commonly operate with wire speeds of 2,500 feet per minute.

The control of rush-drag is the field to which this invention pertains.

2. Prior Art In attempting to control rush-drag, the prior art established that slice velocity could not be measured directly. However, from fluid dynamic considerations, it was perceived that one could obtain an inferential measurement of slice velocity by measuring the fluid head which gave rise to the slice velocity. Once the fluid head had been measured, a signal representative of slice velocity, V, could be obtained through the use of the equation,

v =2gh orv=- 2gh Thus, in the prior art systems, a signal, h, representative of the fluid head was obtained and was put through a square root extractor and, by appropriate scaling of the signals, the multiplication by 2g was simultaneously accomplished with the result that the output from the square root extractor was a signal representative of the slice velocity, V. Similtaneously, the wire speed was measured and compared to the previously obtained slice velocity signal. The difference signal resulting from this comparison was a signal representative of the prevailing rush-drag condition.

SUMMARY OF THE INVENTION Functionally, my control system operates in accordance with a differentiated and rearranged form of the equation,

Thus, differentiating the above equation, one obtains 2v dv 2g dh, or dv (8/ v wherein dv= the rush or drag, in units of speed v the wire speed,

g the gravitational constant, and

dh the differential head, i.e., that part of the total head which gives rise to the velocity difference between the slice velocity and the wire speed, in units of inches of H 0.

My control system utilizes the above differentiated and rearranged equation as follows.

Means are provided for generating a signal representative of the fluid head which gives rise to the slice velocity.

Additionally, means are provided for generating a signal which is speed related to the fluid head which gives rise to the slice velocity. In the preferred embodiment of my invention, the last mentioned means could be a tachometer operatively connected to the fourdrinier belt in which case the speed signal produced would be representative of the actual wire speed. Alternatively, from the wire speed control loop, one could utilize the wire speed set point signal which, presumably, would equal the actual wire speed. As another alternative, means could be provided for generating a signal representative of the desired slice velocity.

Irrespective of the nature of the particular speed signal utilized, the speed signal is squared as opposed to the approach utilized by the prior art wherein the fluid head signal was subjected to a square root operation.

After appropriate scaling and signal conversion, the square of the speed signal is compared with the fluid head signal to obtain a difference signal which, physically, is representative of the differential head, i.e., that part of the total head which corresponds to the rush-drag velocity. The differential head signal is divided by the speed signal thus providing a signal which is representative of the actual rush-drag. Means are then provided for comparing this latter signal to a rush-drag set point to obtain a second difference signal. Control means are provided for controlling the rush-drag in response to this second difference signal.

DESCRIPTION OF DRAWINGS The FIGURE is a functional, schematic representation of my control system and the appropriate papermaking apparatus associated therewith.

DETAILED DESCRIPTION OF THE INVENTION Referring to the FIGURE, the preferred embodiment of my invention, there is shown therein a paper machine headbox 16 containing an aqueous pulp slurry or stock 17. Stock pump 12 pumps the stock to the headbox 16 through conduit 1 1. While there are many approaches available to control stock flow to a paper machine headbox, the apparatus arrangement of the FIGURE contemplates that the stock flow in the conduit 11 to the paper machine headbox 16 will be controlled by manipulating the speed of the stock pump 12.

The particular headbox 16 depicted in the FIGURE is of the closed or pressurized variety. That is to say, there is an air pad or high pressure zone 15 maintained in the headbox above the stock 17. Further, in the particular configuration shown in the FIGURE, the air pressure in the zone 15 is utilized to control the level of the stock 17 within the headbox 16. Thus, pressure measurements are made at 20 and 21 to obtain a differential pressure signal indicative of the level of the stock 17 in the headbox. The two pressure measurements thus made are provided to a level control system 22 which provides a level control signal 23. The level control signal 23 is used to actuate a valve 25 through the valve actuator 24. The valve 25 controls the flow of air through the conduit 26 thus controlling the pressure in the zone 15. The details of the level control system have not been shown in the FIGURE, since such systems are well known to those skilled in the paper making art.

The stock 17 in the headbox 16 is discharged as at 18, onto the moving, fourdrinier wire 19. At point 30 there is provided a speed takeoff to tachometer 31. Tachometer 31 is a standard, commercially available transducer having an input shaft and means for providing an electrical output signal proportional to the angular velocity of the input shaft. Through appropriate gearing and knowing the diameter of the rotating means supplying the input angular velocity, the output of the tachometer will be proportional to the linear velocity or actual wire speed of the fourdrinier belt 19. The electrical output of the tachometer 31 may be either digital or analog depending upon the particular transducer which is selected. The embodiment shown in the FIGURE contemplates that the output signal 32 of the tachometer 31 will be an electrical analog signal, e.g., to 50 ma. The output signal 32 which is representative of wire speed is divided into two equal parts, 32a and 32b, both of which are equal to the signal 32.

Signal 32a is utilized in a local speed control loop. Thus, a set point station 63 provides a signal 64 which is representative of the desired wire speed. The wire speed set point signal 64a, (signal 640 equals signal 64) and the signal 32a representative of the actual wire speed are compared by summing junction 65, thus providing a difference signal 66 which is transmitted to a wire speed controller 50. Wire speed controller 50 may be a standard analog controller which produces an output control signal 52 in response to the input difference signal 66. The output signal 52, which is a wire speed control signal, is transmitted to the speed drive unit 51, wherein a device such as a silicon controlled rectifier may be used to control the drive motors in response to the control signal 52. Thus, the speed of the wire is maintained constant and equal to the speed set point 64.

Signal 32b, which is equal to signal 32 and representative of the actual wire speed, is transmitted to one terminal, i.e., terminal 72, of a three-position switch 70. The switch position shown in the FIGURE represents the preferred embodiment of my invention, i.e., the signal being transmitted through the switch 70 is the signal 32b which is representative of actual wire speed. With the switch in the position shown in the FIGURE, the signal 75 would equal signal 32b, i.e., the signal 75 would be representative of the actual wire speed. Signal 75a and 75b are equal to signal 75.

Signal 75a is utilized as an input to element 33 which is a standard, commercially available squarer, i.e., a unit which produces an output signal equal to the square of the input signal. Moreover, by appropriate scaling of the output signal, multiplication by a constant may be achieved. Thus, the output signal 34 from a squarer 33 is adjusted so as to be equal to the square of the signal 75a divided by 2g (g the gravitional constant) and, physically is representative of the wire speed in terms of a fluid head, i.e., h v /2g. Signal 34 is provided to a current to pressure transducer 35 which produces a pressure output signal 38. The current to pressure transducer 35 is a commercially available unit which, typically, would provide an output pressure ranging from 3 to psi for an input current signal of 10 to 50 ma.

At the base of the headbox 16, a pressure tap 29 is provided. Of course, the pressure measured at 29 will be the total head h, i.e., the fluid head giving rise to the slice velocity which is equal to the sum of the pressure generated by the stock 17 in the headbox 16 plus the air pressure maintained in the zone 15. The total head, h, as measured at point 29 is transmitted by a conduit 80 to a pressure transmitter 81. The output 36 of the pressure transmitter 81 is a 3-15 psi signal which is proportional to the pressure measured at 29. The output signal 36 is transmitted to a narrow range differential pressure transmitter 37. Simultaneously, the'pressure output signal 38 from the current to pressure transducer 35 is also supplied to the difierentiator pressure transmitter 37. The differential pressure transmitter 37 is a commercially available transducer which provides an electrical analog output signal proportional to the input differential pressure. Such units are available with great sensitivity. For example, one can obtain differential pressure transmitters with a total range of 10 inches of water or less, i.e., the output signal will vary over its full range for a variation in the differential input signal of 5 to +5 inches of Water.

The electrical analog output signal 39 from the differential pressure transmitter 37 is representative of the differential head since it results from a comparison of the actual total head and the wire speed expressed in terms of fluid head. Having now obtained a signal which is representative of the differential head, dh, it will be recalled that Utilizing the above equation, the signal 39 representing the differential head dh and the signal 75b representing wire speed v, are supplied to element 40 which is a standard commercially available analog dividing unit, i.e., the output signal 41 from the divider 40 is equal to signal 39 divided by signal 751;. Further, as was the case with the squarer element 33, multiplication by a constant can be achieved through appropriate scaling of the output signal. Thus, the output signal 41 from the divider 40 is so scaled as to provide a multiplication by a constant, viz, g. Alternatively, in place of a straight divider unit, a multiplier divider unit could be employed which performs the function (A)(B)/C. With this latter approach, A could equal g, B would equal signal 39 and C would equal signal 75b. In any case, pure analog dividers or multiplier dividers are commercially available from numerous manufacturers.

Having performed the computation it will be appreciated that the signal 41 is equal to dv, i.e., signal 41 is representative of the actual rush-drag condition. Since a signal has now been obtained which is representative of the actual rush-drag, a comparison can now be effected with a rush-drag set point signal. Thus, there is employed element 42 which is a standard, commercially available analog set-point station which would provide an output signal (rushdrag set-point) 43 compatible with the rest of the system, e.g., 10 to 15 ma, which is compared with signal 41 by summing junction 44. The output signal 45 from the summing junction 44 is equal to the difference, if any, between the actual rushdrag signal 41 and the desired rush-drag represented by the rush-drag signal 43. Having obtained a rush-drag error or difference signal, viz, signal 45, a number of approaches may be utilized to effect control of rush-drag. For example, in my preferred embodiment as shown in the FIGURE, the error signal 45 is applied to a standard analog controller 46 which, preferably, would be a two action controller. The output or control signal 47 from the controller 46 would be utilized to alter the speed of the stock pump 12 and thus alter the total head resulting in a change in the slice velocity in order to reestablish the correct rush-drag condition.

As heretofore pointed out, the preferred embodiment of my invention contemplates that the speed related signal or speed signal will be the actual wire speed, i.e., as shown in the FIGURE the signal 32 B. As an alternate embodiment, the wire speed set point signal could be utilized in which case the pole 74 of the switch 70 as shown in the FIGURE would be positioned to contact terminal 71. Assuming that the speed control loop was performing its function, i.e., maintaining the actual wire speed equal to the desired wire speed, it will be appreciated that the actual wire speed will equal the wire speed set point and thus the remainder of the control system will perform as previously described. However, it is desirable to have available the alternate approach of utilizing a signal other than the output of the tachometer 31 since either the tachometer 31 or another component of the wire speed control loop may fail in which case the wire speed could be set at a fixed value and the output of the wire speed set point unit 63 could be utilized by placing the switch 70 in position 71.

Still another embodiment of my invention could be realized by placing switch 70 in position 73 wherein terminal 73 is receiving signal 62 from set point station 61. Set point station 61 would be component of the type previously described with respect to set point station 63 except that the signal which is supplied would be a speed signal representative of the desired slice velocity or, otherwise stated, a signal representative of the desired total head but expressed in terms of feet per minute. Thus, when utilizing this embodiment of my invention, if one desired to maintain a total head of 150 inches of water, this total head could be expressed in terms of feet per minute by utilizing the equation v 2gh. If, in actuality, the output 62 of the set point unit 61 is adjusted to provide a slice velocity set point equal to the desired total head, it will be appreciated that the rush-drag set point 43 will be set equal to zero since, under such circumstances, my invention would essentially comprise a total head control system rather than a rush-drag control system.

Having hereinbefore set forth numerous embodiments of my invention, it will be understood that there are still other and further embodiments which will be perceived by those skilled in the art to which this invention pertains. For example, the embodiments of my invention previously described presupposed a substantially electronic control implementation. Of course, my invention as herein disclosed is clearly amenable to a pneumatic implementation. Similarly, although the embodiments of my invention herein described presuppose an analog implementation, those skilled in the art will understand that my invention can be implemented through the use of digital means, i.e., a digital computer programmed to perform the necessary computations, comparisons, and other functions previously described.

lclaim:

l. A system for controlling the velocity of a slurry exiting from a paper machine headbox with respect to the speed of the wire onto which the slurry is deposited which comprises:

a. means for providing a first signal representative of the total head in said headbox;

b. means for providing a speed signal related to said slice velocity;

. means receiving said speed signal and producing a third signal equal to the square of said speed signal multiplied by a constant;

d. means for comparing said first and third signals to produce a first difference signal;

e. means for dividing said first difference signal by said speed signal and multiplying by a constant to produce a fourth signal;

. means for providing a fifth signal representative of the desired difference between slice velocity and said speed signal;

g. means for comparing said fifth signal and said fourth signal to produce a second difference signal; and

h. means for controlling said slice velocity in response to said second difference signal.

2. The system of claim 1 wherein said means for providing a speed signal provides a speed signal which is representative of desired wire speed.

3. The system of claim 1 wherein said means for providing a speed signal provides a speed representative of the desired slice velocity.

4. The system of claim 1 wherein said means for providing a speed signal provides a speed signal representative of actual wire speed.

5. The system of claim 4 wherein said means for controlling comprises:

a. means for generating a control signal in response to said difference signal; and

b. means for controlling the stock flow to the paper machine headbox in response to said control signal.

6. The system of claim 5 which comprises:

a. means for providing a set point signal representative of the desired wire speed; b. means for comparing said set point signal with said signal representative of the actual wire speed to produce a wire speed difference signal; and

c. means for controlling the wire speed in response to said wire speed difference signal.

7. The system of claim 6 wherein said means for controlling the stock flow comprises means for controlling the speed of the stock pump.

8. The system of claim 6 wherein said means for controlling the stock flow comprises means for controlling the bypass flow around the stock pump.

9. An improvement in the method of controlling the velocity of a slurry exiting from a paper machine headbox with respect to the speed of the wire on to which the slurry is deposited including the steps of obtaining a first signal representative of the total head in the headbox and obtaining a second signal representative of the actual wire speed, wherein the improvement comprises the steps of:

a. obtaining a third signal representative of the square of the wire speed signal multiplied by a constant;

b. comparing said third signal to said first signal to obtain a first difference signal;

c. dividing said first difference signal by said second signal and multiplying by a constant to produce a fourth signal;

(1. obtaining a set point signal representative of the desired difference between the slice velocity and the actual wire speed;

e. comparing said set point signal to said fourth signal to obtain a second difference signal; and

f. controlling the stock flow to the headbox in response to said second difference signal. 

2. The system of claim 1 wherein said means for providing a speed signal provides a speed signal which is representative of desired wire speed.
 3. The system of claim 1 wherein said means for providing a speed signal provides a speed representative of the desired slice velocity.
 4. The system of claim 1 wherein said means for providing a speed signal provides a speed signal representative of actual wire speed.
 5. The system of claim 4 wherein said means for controlling comprises: a. means for generating a control signal in response to said difference signal; and b. means for controlling the stock flow to the paper machine headbox in response to said control signal.
 6. The system of claim 5 which comprises: a. means for providing a set point signal representative of the desired wire speed; b. means for comparing said set point signal with said signal representative of the actual wire speed to produce a wire speed difference signal; and c. means for controlling the wire speed in response to said wire speed difference signal.
 7. The system of claim 6 wherein said means for controlling the stock flow comprises means for controlling the speed of the stock pump.
 8. The system of claim 6 wherein said means for controlling the stock flow comprises means for controlling the bypass flow around the stock pump.
 9. An improvement in the method of controlling the velocity of a slurry exiting from a paper machine headbox with respect to the speed of the wire on to which the slurry is deposited including the steps of obtaining a first signal representative of the total head in the headbox and obtaining a second signal representative of the actual wire speed, wherein the improvement comprises the steps of: a. obtaining a third signal representative of the square of the wire speed signal multiplied by a constant; b. comparing said third signal to said first signal to obtain a first difference signal; c. dividing said first difference signal by said second signal and multiplying by a constant to produce a fourth signal; d. obtaining a set point signal representative of the desired difference between the slice velocity and the actual wire speed; e. comparing said set point signal to said fourth signal to obtain a second difference signal; and f. controlling the stock flow to the headbox in response to said second difference signal. 